High energy acrylic prepolymer propellants of low sensitivity

ABSTRACT

High energy propellants of low sensitivity in the range of class 2 military explosives and propellants are disclosed. The propellants are comprised of acrylic copolymers, nitrate ester plasticizers, metallic fuel, inorganic oxidizer, and curatives. The acrylic copolymers are constituted of acrylic acid copolymerized with a nitrated acrylate selected from petrin acrylate and 2,2-dinitropropyl acrylate. The nitrate ester plasticizers are selected from triethylene glycol dinitrate, (TEGDN), trimethylolethane trinitrate, (TMETN), and butane-triol trinitrate, (BTTN). The metallic fuel is powdered aluminum, and the inorganic oxidizer is ammonium perchlorate. A commercially available diepoxide, such as dicyclo diepoxy carboxylate, is used as a curing agent. Compatible cross-linking catalyst such as ferric acetylacetonate may be used to decrease the curing time and/or temperature.

@ ite States atent [191 Baldwin et al.

[451 Jan. 15, 1974 [75] Inventors: Mart G. Baldwin, Newtown, Pa;

Paul H. Gehlhaus, Huntsville, Ala.

[73] Assignee: The United States of America as represented by the Secretary of the Army, Washington, DC.

[22] Filed: Jan. 5, 1970 [21] Appl. No.: 1,573

[52] US. Cl 149/19, 149/20, 149/21, 149/88, 149/89, 149/90, 149/93, 149/114 [51] Int. Cl C0611 5/06 [58] Field of Search 149/19, 20, 21, 88, 149/93, 89, 90, 114

[56] References Cited UNITED STATES PATENTS 3,092,526 6/1963 Philipson 149/19 3,068,129 12/1962 Schaffel l l 149/19 3,389,026 6/1968 Johnson 149/19 3,427,295 2/1969 Reed 149/19 X Primary ExaminerBenjamin R. Padgett Attorneyl-larry M. Saragovitz, Edward J. Kelly, Herbert Berl and James T. Deaton [5 7] ABSTRACT High energy propellants of low sensitivity in the range of class 2 military explosives and propellants are disclosed. The propellants are comprised of acrylic copolymers, nitrate ester plasticizers, metallic fuel, inorganic oxidizer, and curatives. The acrylic copolymers are constituted of acrylic acid copolymerized with a nitrated acrylate selected from petrin acrylate and 2,2- dinitropropyl acrylate. The nitrate ester plasticizers are selected from triethylene glycol dinitrate, (TEGDN), trimethylolethane trinitrate, (TMETN), and butane-trio] trinitrate, (BTTN). The metallic fuel is powdered aluminum, and the inorganic oxidizer is ammonium perchlorate. A commercially available diepoxide, such as dicyclo diepoxy carboxylate, is used as a curing agent. Compatible cross-linking catalyst such as ferric acetylacetonate may be used to decrease the curing time and/or temperature.

4 Claims, N0 Drawings HIGH ENERGY ACRYLIC PREPOLYMER PROPELLANTS OF LOW SENSITIVITY BACKGROUND OF THE INVENTION A calculated specific impulse in the range of 260 sec. has been reached by double-base propellants known in the propellant industry. The sensitivity, ease of processing, and limitations as to range of feasible compositions of high impulse double-base propellants are a number of areas where improvements have been desired.

Low sensitivity composite propellants having high specific impulse are used extensively in tactical rockets. High solids loadings, 85-90 percent, are required to achieve the high specific impulse. The high solids loadings have led to difficulties in processing and to poor reproducibility in some cases. Also the binders, principally polybutadiene, are not amendable to wide variations in properties because of the limited solubility in polybutadiene of many plasticizers of potential interest.

An object of this invention is to provide versatile high performance propellants of low sensitivity and easy processing for use in tactical rockets.

Another object is to provide propellant formulations which have high binder levels and properties which can be varied over a wide range through proper selection of acrylates in the polymer.

A further object is to provide high energy propellants of low sensitivity which employ acrylic copolymers which are compatible with many plasticizers.

SUMMARY OF THE INVENTION The acrylic copolymers of this invention are composed of from 3-10 percent by weight acrylic acid copolymerized with 90-97 percent by weight of a nitrated acrylate such as petrin acrylate or the nitro acrylate, 2,2-dinitropropyl acrylate. A plasticizer, selected from the nitrate ester plasticizers consisting of triethylene glycol dinitrate, (TEGDN), trimethylolethane trinitrate, (TMETN), and butane-trio] trinitrate, (BTTN), may be used in propellant compositions of this invention in a proportion of one to four parts of plasticizer to one part acrylic copolymer. The combined total of copolymer and plasticizer is mixed with the other propellant ingredients, powdered aluminum metal fuel, inorganic oxidizer ammonium perchlorate, dicyclo diepoxy carboxylate (curing agent), and optionally, a cross-linking catalyst, to form an uncured propellant mix. The mix is easily cast into rocket motors to give casebonded propellant grains after being cured at about 140F for 16 to 40 hours.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The acrylic copolymers of this invention are prepared by a procedure illustrated by the polymerization technique for petrin acrylate and acrylic acid to follow.

A solution of 95.0 grams of petrin acrylate, 5.0 grams of acrylic acid, and 1.0 gram of benzoyl peroxide in 300 cc of an inert organic solvent (e.g., ethyl acetate) is prepared and heated under reflux conditions for sixteen hours, during which period complete polymerization occurs. The copolymers are precipitated in hexane. The copolymers are soluble in commonly used organic solvents (e.g., acetone, diethyl ether, ethyl acetate, and the like). When the copolymer is to be used immediately in a propellant mix, the inert organic solvent may be removed after the plasticizer addition is made by stripping the solvent under vacuum.

The propellant compositions of this invention are comprised of a binder (the binder is comprised of a selected amount of acrylic copolymer of about 1 part and a proportion of plasticizer from 1 to 4 parts to each part of acrylic copolymer) in the amount of about 20 to 50 percent by weight of the propellant. Powdered aluminum of 550 micron average particle diameter is used as the metallic fuel in amounts of 5-25 percent by weight of the propellant composition. The inorganic oxidizer is ammonium perchlorate in amounts of 35 to percent by weight of the propellant composition. The diepoxide curing agent, dicyclo diepoxy carboxylate, is used as the curing agent for the propellant. The amount of diepoxide used is a function of the acrylic acid content with the epoxide to acid ratio used varying from a molecular equivalence of l/ l to 2/1 depending upon the final physical properties desired. A crosslinking catalyst such as ferric acetylacetonate may be used to decrease the curing time and/or temperature at which curing is achieved.

A typical propellant composition is set forth under Example I below.

Ferric acetylacetonate (catalytic curing agent) .I I

The propellant set forth under Example I has a calculated standard specific impulse of 263 seconds while the sensitivity is maintained in the range of class 2 military explosives and propellants.

The propellants of this invention may be mixed in a standard vertical blade propellant mixer at a water jacket temperature of l00l40F. The mixing may be accomplished under vacuum, or deaeration may be performed separately after mixing. The uncured propellant mix has a viscosity of 1-3 kilopoise at l30F and is easily cast into rocket motors to give casebonded propellant grains when cured.

Curing of the propellants of this invention is normally accomplished at l40F during a period of time of 16 to 40 hours, but the curing may be varied down to a temperature range of F with curing time periods up to one week depending upon the curing agent and catalyst concentration employed. The properly cured propellant is a rubbery solid with qualitatively attractive mechanical properties.

Examples HA and IIB, below, set forth typical values of certain properties (e.g., sensitivity and burning rates). The sensitivities achieved are well within the range for class 2 military explosives and propellants. The values set forth in examples HA and IIB are based on the composition of Example I propellant employing ammonium perchlorate of the types specified in the respective examples.

EXAMPLE ll A Card Type inorganic Oxidizer Used gap Sensitivity Values Ammonium Perchlorate .l5-.l8 inches (unground) Ammonium Perchlorate .55.60 inches (8 micron) EXAMPLE ll B Type inorganic Oxidizer Used Burning Rates at 1000 psi Ammonium Perchlorate 0.4 inches/sec. (unground) Ammonium Perchlorate l.2 inches/sec. (8 micron) The unground perchlorate is a commercial grade product prepared by crushing crystals of ammonium perchlorate to yield a product of particle size of about 400 microns. The finer grades of ammonium perchlorate are made by grinding and separating to the desired particle size. Various methods of grinding and separating ammonium perchlorate are well known in the propellant manufacturing art.

We claim:

1. A solid propellant composition characterized by having a high energy content such that a high specific impulse is attainable while maintaining a low sensitivity level in the range of sensitivity for tactical military explosives and propellants, said composition being comprised of nitrate ester plasticizers selected from the group consisting of triethylene glycol dinitrate, trimethylol ethane trinitrate, and butane-trial trinitrate, metallic fuel of aluminum metal, inorganic oxidizer of ammonium perchlorate, and an acrylic copolymer; said acrylic copolymer being comprised of from 3-10 weight percent acrylic acid and 90-97 weight percent of a nitrated acrylate selected from the group consisting of petrin acrylate and 2,2-dinitropropyl acrylate; said plasticizers selected being used in said composition in proportions of from about one to about four parts to about one part of said acrylic copolymer to make up the propellant binder of said composition, said binder being used in an amount from about 20 to 50 percent by weight of said propellant composition.

2. The solid propellant composition of claim 1 and wherein said metallic fuel of aluminum metal is powdered and has an average particle size diameter from about 5 to about 50 microns, and said powdered alumi num metal is used in amounts from 5 to 25 percent by weight of said composition; said ammonium perchlorate being selected from unground and 8 micron particle size, and said ammonium perchlorate being present in amounts from 35 to percent by weight of said composition.

3. The solid propellant composition of claim 2 and wherein aid ammonium perchlorate selected is unground and is present in an amount of about 50 percent by weight of said composition; said selected acrylic copolymer is comprised of said petrin acrylate of about parts and of said acrylic acid of about 5 parts, said acrylic copolymer is present in an amount of about 10.87 percent by weight of said composition; said aluminum metal is present in an amount of about 15.00 percent by weight of said composition; said plasticizers are comprised of triethylene glycol dinitrate of about 10.87 percent by weight of said composition and of trimethylolethane trinitrate of about 10.87 percent by weight of said composition; and additionally comprising a curing agent dicyclo diepoxy carboxylate in the amount of about 2.28 percent by weight of said composition and a curing agent, ferric acetylacetonate, in the amount of about 0.1 1 percent by weight of said composition.

4. The solid propellant composition of claim 3 and wherein said ammonium perchlorate selected is eight micron particle size. 

2. The solid propellant composition of claim 1 and wherein said metallic fuel of aluminum metal is powdered and has an average particle size diameter from about 5 to about 50 microns, and said powdered aluminum metal is used in amounts from 5 to 25 percent by weight of said composition; said ammonium perchlorate being selected from unground and 8 micron particle size, and said ammonium perchlorate being present in amounts from 35 to 75 percent by weight of said composition.
 3. The solid propellant composition of claim 2 and wherein said ammonium perchlorate selected is unground and is present in an amount of about 50 percent by weight of said composition; said selected acrylic copolymer is comprised of said petrin acrylate of about 95 parts and of said acrylic acid of about 5 parts, said acrylic copolymer is present in an amount of about 10.87 percent by weight of said composition; said aluminum metal is present in an amount of about 15.00 percent by weight of said composition; said plasticizers are comprised of triethylene glycol dinitrate of about 10.87 percent by weight of said composition and of trimethylolethane trinitrate of about 10.87 percent by weight of said composition; and additionally comprising a curing agent dicyclo diepoxy carboxylate in the amount of about 2.28 percent by weight of said composition and a curing agent, ferric acetylacetonate, in the amount of about 0.11 percent by weight of said composition.
 4. The solid propellant composition of claim 3 and wherein said ammonium perchlorate selected is eight micron particle size. 